Not quite right. If the permitted gross weight of the combination exceeds3 500kg, then a tacho will have to installed and used. But only when the trailer is being towed. Doesn’t matter if the trailer is empty or loaded, ‘trailer on’ = tacho required. On those days where you do some other work with the 4x4 in the morning and tow the trailer in the afternoon, you are required to keep a record for the whole day; either use the tacho all day or make a manual record on the reverse of the chart. Remember, driving a vehicle not subject to EC Regs is DUTY, not driving.
If the work done in the 4x4 in the morning is for hire or reqard, then don’t forget you come under the UK domestic driving regs, in which case you can only drive for 10 hours per day. This time has to be taken into account later in the day when you put a tacho in, as you cannot exceed your total number of hours driving time, which is the combinatiuon of time spent driving under UK and EU regs. You also need to keep a record of it.